The electroluminescent phenomenon was first discovered in 1930s, and the luminescent material was ZnS powder. The LED technology was developed through this and now it is widely used in the energy-saving light source. However, the organic electroluminescent phenomenon was first discovered by Pope et al. in 1963. They found that the anthracene single crystal could emit a faint blue light under the drive of the voltage above 100V. Until 1987, by adopting sandwich device structure, Dr Ching W. Tang et al. of Kodak made a double layered device through vacuum evaporation of fluorescent dyes in the U.S. Pat. No. 4,356,429. The brightness of this device could reach 1000 cd/m2 under the driving voltage of 10 v. This greatly promoted the research of OLED materials and devices.
Compared with inorganic luminescent material, there are the following advantages of the organic electroluminescent material: 1. The processing properties of organic material are good, which could be made into films on any substrate by means of evaporation or spinning, etc.; 2 Due to the diversity of the organic molecular structure, the improvement of the material could be achieved by the design and modification of the structure of the molecular to adjust the heat stability, mechanical properties, luminescence and electric conductivity of the material.
The luminescence principle of electromechanical emitting diodes is similar to that of inorganic luminescent diodes. Its principle is as follows: under the action of the electric field, Hole and Electron are injected respectively through the anode and cathode, the luminescence composite layer form excitons and then the excitons return to ground state by luminescence relaxation, thus the goal of luminescence is achieved.
Most of the material used in the organic electroluminescent device is pure organic material or the mixture of the organic and mental material, which could be divided into hole injection material, hole transport material, luminescent material, electron transport material, electron injection material etc. according to function. Thereinto, the organic material with p-type properties easily being oxidized and having electrochemical stability after being oxidized is often used as hole injection or hole transport material. On the other hand, the organic material with n-type properties easily being reduced and having electrochemical stability after being reduced is often used as electron injection material. The organic material with both p-type and n-type properties is preferably being used as luminescent material.
In order to achieve breakthrough in the application of organic electromechanical device, it is necessary to overcome the difficulties of low capacity of charge injection and transport. An efficient and long-lived organic electroluminescent device is usually the result of the structure of the device and the optimization of various organic materials. This provides a great opportunity and challenge for chemists to develop functionalized materials of various structures.